Conventional video projectors comprising a light valve are currently capable of generating images having a contrast of between 500:1 and 1000:1 depending on the valve technology employed. In certain applications, for example digital cinema or top-of-the-range TV sets, this contrast value is not always sufficient. In order to increase this value, a known solution is to modulate the intensity of the light delivered to the valve depending on the contents of the image to be displayed. For example, if the image to be displayed shows a dark scene, the light intensity delivered to the valve is reduced whereas the level of the video signal processed by this same valve is increased in the same proportion. The contrast is now better since the number of bits in the video signal is effectively increased. It is thus possible to achieve a high contrast for the low grey levels which are always critical in TV applications.
One of the known techniques for modulating the light intensity consists in detecting the peak grey level NGmax in the image to be displayed and in comparing this with the maximum grey level that can be displayed NGMAX (=255 if the levels use 8-bit encoding):                if the grey level NGmax is below half the grey level NGMAX, the intensity of the light delivered to the valve for the image under consideration is divided by 2 and the amplitude of the video signal delivered to the control circuit of the valve is multiplied by 2,        if the grey level NGmax is above half the grey level NGMAX, the intensity of the light delivered to the valve remains at its peak value and the level of the video signal delivered to the control circuit of the valve remains unchanged.        
This technique is illustrated in FIGS. 1A, 1B and 1C. FIG. 1A shows the video signal as a function of time of two images displayed during frames T and T+1, respectively. This signal is delivered to the control circuit of the valve. The voltage of the level NGmax of the first image is lower than the voltage of the level NGMAX/2 and that of the level NGmax of the second image is higher than the voltage of the level NGMAX/2. FIG. 2B shows the light intensity (luminance) delivered to the valve for each of the two images. According to the process previously defined, it is equal to Lmax/2 for the first image and to Lmax for the second image. The voltage of the video signal of the first image is therefore multiplied by 2 and that of the second image is kept as it is. The rendering of the video levels of dark images is thus enhanced.
This technique presents many drawbacks. The first one of them is that the image contrast is not enhanced whenever an image pixel exceeds NGMAX/2. Accordingly, if the image comprises a single luminous point over a dark background, the image contrast is not increased.
In addition, there is a high current demand (during the transition from Lmax/2 to Lmax or vice versa) within the light source each time there is a transition from an image having a grey level NGmax below NGMAX/2 to an image having a grey level NGmax higher than NGMAX/2 or vice versa. Finally, the device responsible for modulating the light delivered to the valve is not able to switch instantaneously from Lmax/2 to Lmax or vice versa. Consequently, during the transition, the video signal level cannot be correctly adjusted so that areas of blurred image appear during these transition periods.
The invention proposes an image processing method that allows all or part of the above-mentioned drawbacks to be dealt with.